One such type of method is known from DE 32 26 818 A1. During this known method conventional devices are used for the measurement of gas and gas loading. After a measurement of density of a mixture sample which was taken from a machine container, or in a bypass the gas loading done at another position is controlled on the basis of this measurement. The addition of the gas occurs also in a bypass, for example, parallel to a return pipe-line coming from a mixer casing in the vicinity of a machine tank. The gas addition in the bypass is done by a capillary tube introduced into the pipe-line and a Venturi-tube. The control of gas loading can be done manually or with the help of a computer. The application of the Venturi-principle makes this known method difficult to control because the amount of gas loading is heavily dependent on an interface tension between liquid and gas, wherein high interface tension means small bubbles of the absorbed gas in the liquid and wherein low interface tension means large bubbles of the absorbed gas. In addition to that the known method working according to the Venturi-principle is high energy consuming because the flow in the pipe, in which the Venturi-device is situated, must have a high speed, so that the gas loading resulting according to the type of a water jet blast can function at all. Finally it is disadvantageous in the case of the known method, that relatively high quantities of foaming agent, among others hydrocarbon fluids, are necessary.
Another known method is, for example, mechanical frothing. In this case foaming agent, air and gelling agent are fed to a plastic serving as a skeleton substance and a homogeneous mixture is produced by a mixer (vide A.M. Wittfoth, Kunststofftechnisches Worterbuch, Carl Hanser Verlag Munchen Wien, Part 3, page 146, 1978). The mixture is then poured into a mould. In a subsequent heating operation the mixture expands and solidifies, so that a foamed product is produced. Therein a foaming agent in the form of azo or diazo-compounds is always added, which sets free N.sub.2 or CO.sub.2, when the foaming agent decomposes under gas formation in the heating operation subsequent to the mixing and thus leads to the foaming up of the skeleton substance and to the formation of foam. In the case of some foaming agents no heating process is necessary, as they decompose even at room temperature. However, even here normally a heating process is conducted afterward in order to cure the foam.
Again there are other methods for the production of foam from plastic, e.g. the one-shot process, which is also called direct process, or the two-step process. Therein either foaming agents or liquid solvents are admixed to a plastic to be foamed up. The foaming agents decompose at a certain temperature under gas formation and thus foam up the plastic by forming cavities within the plastic. In the case of production of foams by adding volatile solvents these are admixed to the plastic during the polymerisation, whereby spaces filled with solvent are created, which become cavities after the evaporation of the solvent.
Foams are produced from polystyrene, polycarbonates or polyurethanes, for example, wherein the production of foams from polyurethanes has the greatest technical importance and such foams find use in upholstery, mattresses or wrappings, for example.
Most of the known methods have the disadvantage, that during the use of foaming agents free from chlorofluorocarbons (CFC), like n-pentane, for example, with air there are easily produced very highly inflammable air-gas mixtures. These air-gas mixtures develop not only in the production phase of the foams, but also during aging of the foams afterward, during which the residual foaming agent is set free from the foam. Production companies for foams which handle such types of foaming agents, therefore require extremely high safety precautions. This is also valid for the later aging in rooms, in which the permitted concentration of gas density must be permanently checked and regulated by constant ventilation of the rooms. This requires not only expensive, matured technology within the production company, but leads also to a very high emission load of the atmosphere.
In the not pre-published DE 41 13 578 A1, which originates from this applicant, a device for the loading of viscous liquids with gases, in particular polyols for the production of polyurethane with preferably carbondioxide is described. If only carbondioxide is applied as gas for the foaming of the plastic during the production of foam, it can result to the formation of urea. This urea then leads to brittleness of the foam. Integral skin foam cannot be produced by this known device, where only CO.sub.2 or any other similar gas is applied for foaming.